Add a pre-commit hook to check that pre-executed notebooks are marked so #558

It is tricky to run manual documentation builds on PPT-hydrated projects, because notebooks in pre_executed should be handled separately. To help people keep the notebooks in the right state, we can ensure that they contain metadata.nbsphinx.execute: "never":
https://nbsphinx.readthedocs.io/en/0.9.8/never-execute.html
https://raw.githubusercontent.com/spatialaudio/nbsphinx/refs/heads/master/doc/never-execute.ipynb

Here is an example of such a hook in the coniferest repo:
https://github.com/snad-space/coniferest/blob/eea28212644875ba2a50e7afed1ca27b49fab229/.pre-commit-config.yaml#L33-L38
https://github.com/snad-space/coniferest/blob/eea28212644875ba2a50e7afed1ca27b49fab229/.ci/pre_executed_nb.py